Transistor help

Hi, i'm currently a graduate level physics student taking a course in analog electronics, and i'm really stuck on the idea of transistors. I am reading "the art of electronics", and it's explanation of transistors is horrible, and aside from the fact that i know they "magically" cause a gain in current (still not sure why), I have no idea what is going on inside to cause this magic (the book doesn't feel like explaining this to me either.) I was wondering if someone can illuminate me with the physics behind the transistor, thank you.

Staff: Mentor

Yeah, The Art of Electronics is not going to explain the physics behind transistor action -- that's not what that book is for. Have you read the wikipedia.org explanation? What other resources have you tried? What is the course you are taking, and what is the textbook?

BJT action is not easy to understand or express mathematically, so here is a short version.

The modus operandi is that of two p-n semiconductor junctions in *very close proximity* to one another. Let's use an npn device for this example. The base-emitter, or b-e junction gets forward biased, whereas the base-collector, or b-c junction gets reverse biased. Under these conditions an electric field exists in the b-c region such that positive charge in the said region would move from the collector to the base, and opposite for negative charge. In the b-e region, an electric field is also present such that positive charge will move from base to emitter and vice-versa for negative charge.

Because the collector is n-type material, and the base is p-type, very little collector current, only leakage Icbo, exists when the b-e junction is not forward biased, as the b-c jcn is reverse biased. When the b-e jcn is fwd biased, holes (positive charge) move from the base towards the emitter, and electrons (negative) move from the emitter towards the base. If the base region is very very thin, the following will occur. The electrons that have just been emitted by the emitter will enter the base region and encounter the strong electric field associated with the reverse biased b-c junction. The polarity of said field is such as to attract electrons from the base region into the collector. Thus emitter electrons cross the base region into the collector and become collector current.

If the forward bias on the b-e jcn is a time-varying, or *ac* signal, the emitter current will be time-varying as well. The motion of electrons from emitter towards base will vary in time with the ac signal at the b-e jcn. Likewise, these electrons will be attracted into the collector region as a function of time in accordance with the input signal. Thus the collector current is an amplified facsimile of the input signal.

What makes active devices (bjt, FET, vacuum tube, IGBT, etc.) so useful is that they provide both current and voltage gain. The signal variation at the b-e jcn consists of a small current and small voltage. By intentionally doping the p-type base with a much lower density of acceptor atoms vs. the n-type emitter doping with a high density of donor atoms, the hole density from base to emitter is quite small compared with the electron density from emitter to base, or base current Ib << emitter current Ie. Also, due to the logarithmic diode nature of the forward biased b-e p-n junction, the base to emitter voltage swing, Vbe, is very small compared with the collector terminal voltage swing. Thus a bjt provides a large value of both current gain and voltage gain.

To summarize, when the b-e jcn is forward biased, charges emitted from the emitter are drawn into the collector due to the strong attraction of the electric field present in the reverse biased b-c junction. By using light doping in the base, and heavy doping in the emitter, large values of current gain can be realized. Because of the diode junction formed at the b-e region, large current swings are accompanied by small voltage swings due to the logarithmic nature of the p-n junction. Large values of voltage gain are attainable as a result.

Transistors came from crystal-set radios. The crystal set detector consisted of a lump of crystalline galena (ZnS) with the end of a wire poking onto the surface of the crystal. You had to move the end of the wire (cat whisker) around on the surface to find a “hot spot”. When you found the hot spot, you could hear the music in your ear phones.

When touching the hot spot, the connection between the wire (conductor) and the crystal (semiconductor) is nonlinear and acts like a diode.

The old time experimenters knew that the current could flow FROM* the cat whisker to the galena, but not TO the cat whisker from the galena, I.e., diode action. They wondered what would happen with two cat whiskers on the same hot spot. If they tried to push current into the galena with one cat whisker, what would happen with other?

They found out that if you try to push current to the galena with one cat whisker (back bias) and at the same time pull current from the galena with the other cat whisker (forward bias), the current passes from one cat whisker to the other with very little going into the galena!

Of course, that small amount the goes into the galena has to exit through its mounting connection on the crystal set’s chassis. After thinking about it awhile, they realized that they could control the current in the whisker-whisker circuit by controlling the small amount of current exiting the galena. This is current amplification.

Since the galena crystal formed a base to support the cat whiskers, now-a-days we call this element of the transistor the Base. The forward-biased whisker “emitted” current into the galena, so it was named the Emitter. The reversed-biased whisker collected the current the was intended to go into the galena, so it’s called the Collector in modern transistors.

*I don’t know if galena is n-type of p-type, so maybe current really flow TO the galena and FROM the whisker.

Basic Electronics by _____. This book will explain things without diving too deep into it...I found it usefull in highschool, but the sedra smith book is much more advanced and gives a more detailed and sophisticated explanation.